The present invention relates to a starter for a gas discharge lamp, in particular a high-pressure gas discharge lamp for motor vehicle headlights.
In the case of a starter circuit of this type known from German Patent Application No. 44 23 275, the starting voltage is generated with the help of a cascade circuit which increases the voltage, but there is also a d.c./a.c. converter connected upstream from the cascade. The expense of operating the starter in this case should not be underestimated.
In addition, it is known from German Patent No. 40 17 415 that a voltage may be supplied to the primary winding of a starter transformer in a starting device. The secondary winding of this starter transformer then transforms this voltage up to the voltage needed to start the lamp. On the primary side, a capacitor is arranged in parallel with the series connection of the primary winding and a controllable switch such as a controlled thyristor. This capacitor is charged to an effective voltage which is then applied at this point for the purpose of starting. On reaching a certain voltage, the controllable switch is enabled and the capacitor is discharged. With this known starting device, a very high additional voltage is needed over a separate voltage source to charge the capacitor.
The starter according to the present invention for a gas discharge lamp has the advantage that the required starting voltage is supplied easily, is achieved with very inexpensive components, and the starter is supplied with power directly by the control circuit, i.e., the bridge circuit within the control circuit. This simplifies the starter while reducing the inductance of the starter. This also yields an improvement in radiant emittance. In addition, the present invention makes it possible to integrate the starter and the controller, thus permitting a further reduction in cost and space.
This is achieved according to the present invention by the fact that a cascade circuit of capacitors and diodes is provided in the starter circuit to generate the starting voltage at a charge capacitor; the starter circuit is supplied with power by the bridge circuit which supplies the gas discharge lamp with power for operation; the bridge circuit for starting is cycled at a frequency which is much higher than the normal frequency for lamp operation, and the bridge circuit is switched to the normal frequency for lamp operation after the lamp has been successfully started.
According to especially expedient and advantageous embodiments of the present invention, in principle three different embodiments are possible. This may be a two-wire starter or a three-wire starter or a four-wire starter.
In the case of the advantageous embodiment as a two-wire starter, the starter is connected to the controller with two lines. In an advantageous embodiment and an expedient refinement of this embodiment, a switching element, in particular a spark gap, a transistor or a thyristor, is provided in series with the primary winding of the transformer to generate the starting pulse on reaching the starting voltage on the primary side of the transformer.
In the especially expedient and advantageous embodiment of the starter as a three-wire starter, the starter is connected to the controller by three lines, with one terminal of the primary winding of the transformer being connected directly to the controller without an intermediate switching element, and the other terminal of the primary winding being connected to the output of the cascade. This refinement provides increased safety plus the possibility of integrating the starter and controller.
In another advantageous embodiment of this version of the starter, in the controller the one directly connected terminal of the primary winding of the transformer is short-circuited electronically or mechanically to the instrument ground by a switch or a relay provided there to initiate starting in a controlled manner.
In the case of the advantageous embodiment and refinement of the present invention in the four-wire starter version, the starter is connected to the controller by four lines, where the one terminal of the primary winding of the transformer is connected to the reference ground of the controller by way of a controllable switch, like the one terminal of the charge capacitor, while the other terminal of the primary winding is connected to the output of the cascade, and the fourth line is the control line for the controllable switch. In an expedient embodiment, a relay or a transistor or a thyristor may be used as the controllable switch.
According to an advantageous refinement of the present invention relating to all possible embodiment versions, the two-wire version, the three-wire version or the four-wire version, for starting the bridge circuit of the controller is operated at a frequency of approx. 20 kHz, and after the lamp has been successfully started, it is switched to an operating frequency of approx. 400 Hz.